The present invention relates to spark ignition motor fuel compositions based on liquid hydrocarbons derived from biogenic gases that are blended with a fuel grade alcohol and a co-solvent for the liquid hydrocarbon and the alcohol, and having an anti-knock index, a heat content, and a Dry Vapor Pressure Equivalent (DVPE) effective to fuel a spark ignition internal combustion engine with minor modifications. In particular, the present invention relates to Coal Gas Liquid (CGL) or Natural Gas Liquids (NGLs)-ethanol blends in which the co-solvent is biomass-derived 2-methyltetrahydrofuran (MTHF).
A need exists for alternatives to gasoline motor fuels for spark ignition internal combustion engines. Gasoline is derived from the extracting of crude oil from oil reservoirs. Crude oil is a mixture of hydrocarbons that exist in liquid phase in underground reservoirs and remains liquid at atmospheric pressure. The refining of crude oil to create conventional gasoline involves the distillation and separation of crude oil components, gasoline being the light naptha component.
Only ten percent of the world reserves of crude oil lie in the United States, with an overwhelming majority of the remaining 90 percent located outside the boundaries, not only of the United States, but also its North American free trade partners. Over 50 percent of conventional gasoline is imported, with this number to increase steadily into the next century.
Conventional gasoline is a complex composite of over 300 chemicals, including napthas, olefins, alkenes, aromatics and other relatively volatile hydrocarbons, with or without small quantities of additives blended for use in spark ignition engines. The amount of benzene in regular gasoline can range up to 3-5 percent, and the amount of sulfur to 500 ppm. Reformulated gasoline (RFG) limits the quantity of sulfur to 330 ppm and benzene to one percent, and limits the levels of other toxic chemicals as well.
Conventional alternatives to crude oil-derived fuels such as compressed natural gas, propane and electricity require large investments in automobile modification and fuel delivery infrastructure, not to mention technological development. A need exists for an alternative fuel that provides the combustion properties of motor gasoline without requiring significant engine modification, and that can be stored and delivered like motor gasoline. In order to be an advantageous alternative for gaseous alternative fuels such as methane and propane, liquid alternative fuels should also meet all Environmental Protection Agency (EPA) requirements for xe2x80x9cclean fuels.xe2x80x9d
CGL and NGLs have unsuitably low anti-knock indexes and have thus been under-utilized as alternatives to crude oil as hydrocarbon sources for spark ignition engine motor fuels. Attempts to overcome this deficiency have rendered these hydrocarbon streams unsuitable for use as alternative fuels.
Coal gases have long been recognized because of explosions that have occurred in the course of coal mining. This gas is considered a hazard to operations and has been vented to insure safe operation. However, such venting contributes to the increasing amounts of atmospheric methane, which is a potent greenhouse gas. C. M. Boyer, et al., U.S. EPA, Air and Radiation (ANR-445) EPA/400/9-90/008. Coal gases can contain significant amounts of heavier hydrocarbons, with C2+ fractions as high as 70 percent. Rice, Hydrocarbons from Coal (American Association of Petroleum Geologists, Studies in Geology #38, 1993), p. 159.
In contrast to the sourcing of conventional gasoline, over 70 percent of the world reserves of NGLs lie in North America. Imports of NGLs into the United States constitutes less than 10 percent of domestic production. NGLs are recovered from natural gas, gas processing plants, and in some situations, from natural gas field facilities. NGLs extracted by fractionators are also included within the definition of NGLs. NGLs are defined according to the published specifications of the Gas Processors Association and the American Society for Testing and Materials (ASTM). The components of NGLs are classified according to carbon chain length as follows: ethane, propane, n-butane, isobutane and xe2x80x9cpentanes plus.xe2x80x9d
xe2x80x9cPentanes-plusxe2x80x9d is defined by the Gas Processors Association and the ASTM as including a mixture of hydrocarbons, mostly pentanes and heavier, extracted from natural gas and including isopentane, natural gasoline, and plant condensates. Pentanes-plus are among the lowest value NGLs. While propanes and butanes are sold to the chemical industry, pentanes-plus are typically diverted to low-added-value oil refinery streams to produce gasoline. Part of the reason why pentanes plus are not generally desirable as gasoline is because they have a low anti-knock index that detracts from its performance as a spark ignition engine motor fuel, as well as a high DVPE which would result in engine vapor lock in warm weather. One advantage of pentanes plus over the other NGLs is that it is liquid at room temperature. Therefore is the only component that can be used in useful quantities as a spark ignition engine motor fuel without significant engine or fuel tank modification.
U.S. Pat. No. 5,004,850 discloses an NGLs-based motor fuel for spark ignition engines in which natural gasoline is blended with toluene to provide a motor fuel with satisfactory anti-knock index and vapor pressure. However, toluene is an expensive, crude oil-derived aromatic hydrocarbon. It""s use is severely restricted under the reformulated fuel provision of the 1990 Clean Air Act Amendments.
U.S. Pat. No. 4,806,129 discloses a fuel extender for lead-free gasoline wherein the extender consists essentially of a residue naphtha obtained as a by-product of a basic crude oil refining process, anhydrous ethanol, a stabilizing amount of a water repellant (e.g., ethyl acetate and methyl isobutyl ketone), and aromatics (e.g., benzene, toluene, and xylene). As noted above, however, certain aromatics are undesirable and their use may be restricted by law due to the damaging effects on the environment.
German DE-OS 30 16 481 discloses a fuel additive useful for solubilizing water-containing mixtures of hydrocarbons and alcohols, such as gasoline and methanol. The disclosed additive includes tetrahydrofuran and purportedly may be combined with a mixture of gasoline, methanol, and water to form a stable, clear mixture.
The United States is the world""s largest producer of fuel alcohol, with less than ten percent of ethanol imported. Ethanol is a biomass-derived, octane-increasing motor fuel additive. While ethanol alone has a low vapor pressure, when blended alone with hydrocarbons, the resulting mixture has an unacceptably high rate of evaporation to be used in EPA designated ozone non-attainment areas, which include most major metropolitan areas in the United States. The vapor pressure properties of ethanol do not predominate in a blend with pentanes plus until the ethanol level exceeds 60 percent by volume. However, blends containing such a high level of ethanol are costly and difficult to start in cold weather because of the high heat of vaporization of ethanol. Furthermore, ethanol has a low heat content, resulting in low fuel economy compared to gasoline.
Low-cost production of MTHF and the production and use of biomass-derived materials such as ethanol or MTHF as gasoline extenders at levels up to about ten percent by volume is disclosed by Wallington et al., Environ. Sci. Technol., 24, 1596-99 (1990); Rudolph et al., Biomass 16, 33-49 (1988); and Lucas et al., SAE Technical Paper Series, No. 932675 (1993). Low-cost production of MTHF and its suitability as a low-octane oxygenate for addition to gasoline with or without ethanol to produce an oxygenated motor fuel was disclosed in an unpublished presentation to the Governors""Ethanol Coalition by Stephen W. Fitzpatrick, Ph.D., of Biofine, Inc. on Feb. 16, 1995. Accurate technical data involving the blending DVPE and blending octane values for MTHF were not available. There remains a need for a motor fuel having a DVPE and anti-knock index suitable for use in a spark ignition internal combustion engine without significant modification obtained from non-crude oil sources.
This need is met by the present invention. Co-solvents for CGL, and for NGLs hydrocarbons such as natural gasoline or pentanes plus, and motor fuel alcohols such as ethanol have been discovered that result in a blend having the requisite DVPE and anti-knock index for use in a conventional spark ignition engine with minor modifications.
Therefore, in accordance with the present invention, a spark ignition motor fuel composition is provided consisting essentially of:
a hydrocarbon component consisting essentially of one or more hydrocarbons selected from four to eight carbon atom straight-chained or branched alkanes, wherein the hydrocarbon component has a minimum anti-knock index of 65 as measured by ASTM D-2699 and D-2700 and a maximum DVPE of 15 psi (one atmosphere (atm.)) as measured by ASTM D-5191;
a fuel grade alcohol; and
a co-solvent for the hydrocarbon component and the fuel grade alcohol;
wherein the hydrocarbon component, the fuel grade alcohol and the co-solvent are present in amounts selected to provide a motor fuel with a minimum anti-knock index of 87 as measured by ASTM D-2699 and D-2700, and wherein the fuel composition is essentially free of olefins, aromatics, and sulfur.
Motor fuel compositions in accordance with the present invention may contain n-butane in an amount effective to provide the blend with a DVPE between about 12 (0.8 atm.) and about 15 psi (1 atm.) as measured by ASTM D-5191. The n-butane is preferably obtained from NGLs and CGL.
Another embodiment of the present invention provides a method for lowering the vapor pressure of a hydrocarbon-alcohol blend. Methods in accordance with this embodiment of the present invention blend a motor fuel grade alcohol and a hydrocarbon component with an amount of a co-solvent for the alcohol and the hydrocarbon component so that a ternary blend is obtained having a DVPE as measured by ASTM D-5191 lower than the DVPE for a binary blend of the alcohol and the hydrocarbon component. The hydrocarbon component consists essentially of one or more hydrocarbons selected from the group consisting of four to eight carbon atom straight-chained or branched alkanes. The ternary blend is essentially free of olefins, aromatics, and sulfur.
The co-solvent for the hydrocarbon component and the fuel grade alcohol in both the fuel compositions and methods of the present invention is preferably derived from waste cellulosic biomass materials such as corn husks, corn cobs, straw, oat/rice hulls, sugar cane stocks, low-grade waste paper, paper mill waste sludge, wood wastes, and the like. Co-solvents capable of being derived from waste cellulosic matter include MTHF and other heterocylical ethers such as pyrans and oxepans. MTHF is particularly preferred because it can be produced in high yield at low cost with bulk availability, and possesses the requisite miscibility with hydrocarbons and alcohols, boiling point, flash point and density.
Fuel compositions in accordance with the present invention thus may be derived primarily from renewable, domestically-produced, low cost waste biomass materials such as ethanol and MTHF in combination with hydrocarbon condensates otherwise considered extraction losses of domestic natural gas production such as pentanes plus, and are substantially free of crude oil derivatives. The compositions are clean alternative fuels that contain no olefins, aromatics, heavy hydrocarbons, benzene, sulfur, or any products derived from crude oil. The compositions emit fewer hydrocarbons than gasoline, to help states reduce ozone and meet federal ambient air quality standards. Compositions may be prepared that meet all EPA requirements for xe2x80x9cclean fuels,xe2x80x9d yet at the same time utilize current automobile technology with only minor engine modifications. The compositions require little more than presently existing fuel delivery infrastructure and are based on components that result in a blend that is capable of being competitively priced with gasoline. Other features of the present invention will be pointed out in the following description and claims, which disclose the principles of the invention and the best modes which are presently contemplated for carrying them out.
The above and other features and advantages of the present invention will become clear from the following description of the preferred embodiments considered in conjunction with the accompanying drawings.
The compositions of the present invention are virtually free of undesirable olefins, aromatics, heavy hydrocarbons, benzene and sulfur, making the fuel compositions very clean burning. The fuel compositions of the present invention may be utilized to fuel conventional spark-ignition internal combustion engines with minor modification. The primary requirement is the lowering of the air/fuel ratio to between about 12 and about 13, as opposed to 14.6, typical of gasoline fueled engines. This adjustment is necessary because of the large quantity of oxygen that is already contained in the fuel.
This adjustment can be accomplished in vehicles manufactured in 1996 and thereafter by software modifications to the on-board engine computer. For older cars, it will be necessary to replace a chip in the on-board engine computer or, in some cases, to replace the on-board engine computer entirely. Carbureted vehicles, on the other hand, can be readily adjusted to the appropriate air/fuel ratio, and at most will require a simple orifice replacement. Vehicles fueled by the compositions of the present invention preferably should be adapted to run on ethanol or methanol by having fuel system components installed that are compatible with ethanol and methanol, and do not have parts in contact with the fuel made from ethanol and methanol sensitive materials such as nitrile rubber, and the like.
The Clean Air Act Amendments of 1990 set maximum values for both olefins and aromatics, because they result in emission of unburned hydrocarbons. A maximum of 24.6 percent by volume of aromatics may be present in the winter, and 32.0 percent by volume in the summer. A maximum of 11.9 percent by volume of olefins may be present in the winter, and a maximum of 9.2 percent by volume in the summer. Benzene must be present at a level less than or equal to 1.0 percent by volume, and the maximum permitted sulfur is 338 ppm. The fuel compositions of the present invention are essentially free of such materials.
Motor fuel compositions according to the invention are produced by blending one or more hydrocarbons with a fuel grade alcohol selected from methanol, ethanol and mixtures thereof and a co-solvent for the one or more hydrocarbons and the fuel grade alcohol. The fuel grade alcohol is added to increase the anti-knock index of the hydrocarbon component. The co-solvents of the present invention make it possible to add to the motor fuel compositions significant quantities of alcohol effective to provide an acceptable combination of anti-knock index and DVPE. Suitable fuel grade alcohols can be readily identified and obtained for use in the present invention by one of ordinary skill in the art.
Other anti-knock index increasing additives may be used as well, including those additives, such as toluene, derived from crude oil. However, preferred compositions in accordance with the present invention will be substantially free of crude oil derivatives, including crude oil-derived additives for increasing the anti-knock index.
Essentially any hydrocarbon source containing one or more five to eight carbon atom straight-chained or branched alkanes is suitable for use with the present invention if the hydrocarbon source, as a whole, has a minimum anti-knock index of 65 as measured by ASTM D-2699 and D-2700 and a maximum DVPE of 15 psi (1 atm.) as measured by ASTM D-5191. Those of ordinary skill in the art understand the term xe2x80x9canti-knock indexxe2x80x9d to refer to the average of the Research Octane Number (xe2x80x9cRONxe2x80x9d is xe2x80x9cRxe2x80x9d) as measured by ASTM D-2699 and the Motor Octane Number (xe2x80x9cMONxe2x80x9d is xe2x80x9cMxe2x80x9d) as measured by ASTM D-2700. This is commonly expressed as (R+M)/2.
The hydrocarbon component is preferably derived from CGL or NGLs, and is more preferably the NGLs fraction defined by the Gas Processors Association and the ASTM as xe2x80x9cpentanes plus,xe2x80x9d which is a commercially available commodity. However, any other hydrocarbon blend having an equivalent energy content, oxygen content and combustion properties may also be used. For example, the fraction of NGLs defined by the Gas Processors Association and the ASTM as xe2x80x9cnatural gasolinexe2x80x9d can be blended with isopentane and substituted for pentanes plus. Natural gasoline alone may be used, as well. In most circumstances, the preparation of blends instead of using xe2x80x9cstraightxe2x80x9d pentanes plus or natural gasoline will be more costly. While any other equivalent blend may be used, similar cost considerations apply.
The hydrocarbon component is blended with the fuel grade alcohol using a co-solvent selected to provide a blend with a DVPE below 15 psi (1 atm.) without a sacrifice in the anti-knock index or flash point of the resulting blend, so that a motor fuel composition is obtained suitable for use in a spark ignition engine with minor modifications. Co-solvents suitable for use with the present invention are miscible in both the hydrocarbons and the fuel grade alcohol and have a boiling point high enough to provide a DVPE less than 15 psi (1 atm.) in the final blend, preferably greater than 75xc2x0 C. The co-solvent should have a flash point low enough to ensure cold starting of the final blend, preferably less than xe2x88x9210xc2x0 C. The co-solvent should also have at least an 85xc2x0 C. difference between the boiling point and flash point and a specific gravity greater than 0.78.
Five to seven atom heterocyclic ring compounds are preferred as the co-solvent. The heteroatomic polar ring structure is compatible with fuel grade alcohols, yet possesses non-polar regions compatible with hydrocarbons. The heteroatomic structure also functions to depress the vapor pressure of the co-solvent and consequently the resulting blend. The same advantageous properties can also be obtained from short-chained ethers; however, ring compounds are preferred.
Saturated alkyl-branched heterocyclic compounds with a single oxygen atom in the ring are preferred, because the alkyl branching further depresses the vapor pressure of the co-solvent. The ring compound may contain multiple alkyl branches however, a single branch is preferred. MTHF is an example of a five-membered heterocyclic ring with one methyl branch adjacent to the oxygen atom in the ring.
While nitrogen containing ring compounds are included among the co-solvents of the present invention, they are less preferred because the nitrogen heteroatoms form oxides of nitrogen combustion products, which are pollutants. Thus, oxygen-containing heterocyclic ring compounds are preferred over rings with nitrogen heteroatoms, with alkylated ring compounds being more preferred. In addition, the ring oxygen also functions as an oxygenate that promotes cleaner burning of the motor fuel compositions of the present invention. Thus, oxygen-containing heterocyclic ring compounds are particularly preferred co-solvents in the motor fuel compositions of the present invention because of their ability as oxygenates to provide a cleaner burning fuel composition which is in addition to their being a vapor pressure-lowering co-solvent for hydrocarbons and fuel grade alcohols.
Accordingly, oxygen-containing saturated five- to seven atom heterocyclic rings are most preferred. MTHF is particularly preferred. While MTHF is considered an octane depressant for gasoline, it improves the octane rating of NGLs. Not only does MTHF have superior miscibility with hydrocarbons and alcohols and a desirable boiling point, flash point and density, MTHF is a readily available, inexpensive, bulk commodity item. MTHF also has a higher heat content than fuel grade alcohols and does not pick up water as alcohols do, and is thus fungible in an oil pipeline. This permits larger quantities of the fuel grade alcohols to be used to increase the anti-knock index of the motor fuel compositions.
In addition, MTHF is commercially derived from the production of levulenic acid from waste cellulosic biomass such as corn husks, corn cobs, straw, oat/rice hulls, sugar cane stocks, low-grade waste paper, paper mill waste sludge, wood wastes, and the like. The production of MTHF from such cellulosic waste products is disclosed in U.S. Pat. No. 4,897,497. MTHF that has been produced from waste cellulosic biomass is particularly preferred as a co-solvent in the motor fuel compositions of the present invention. Examples of other suitable co-solvents, selected on the basis of boiling point, flash point, density and miscibility with fuel grade alcohols and pentanes plus, are 2-methyl-2-propanol, 3-butene-2-one, tetrahydropyran, 2-ethyltetra-hydrofuran (ETHF), 3,4-dihydro-2H-pyran, 3,3-dimethyloxetane, 2-methylbutyraldehyde, butylethyl ether, 3-methyltetrahydropyran, 4-methyl-2-pentanone, diallyl ether, allyl propyl ether, and the like. As is readily apparent from the above list, short-chained ethers function as well as heterocyclic ring compounds with respect to miscibility with hydrocarbons and fuel grade alcohols and vapor pressure depression of the resulting motor fuel composition. Like the oxygen-containing heterocyclic ring compounds, short-chained ethers are also ideally vapor pressure-lowering oxygenates.
The motor fuel compositions of the present invention optionally include n-butane in an amount effective to provide a DVPE between about seven (0.5 atm.) and about 15 psi (1 atm.). However, the compositions may be formulated to provide a DVPE as low as 3.5 psi (0.2 atm.). The higher DVPE is desirable in the northern United States and Europe during winter to promote cold weather starting. Preferably, the n-butane is obtained from NGLs or CGL.
The motor fuel compositions also optionally include conventional additives for spark ignition motor fuels. Thus, the motor fuel compositions of the present invention may include conventional amounts of detergent, anti-foaming, and anti-icing additives and the like. The additives may be derived from crude oil; however, preferred compositions in accordance with the present invention are substantially free of crude oil derivatives.
The motor fuel compositions of the present invention are prepared using conventional rack-blending techniques for ethanol-containing motor fuels. Preferably, to prevent evaporative loss emissions, the dense co-solvent component is first pumped cold (less than 70xc2x0 F. (21xc2x0 C.)) through a port in the bottom of a blending tank. The hydrocarbons are then pumped without agitating through the same port in the bottom of the tank to minimize evaporative loss. If used, n-butane is pumped cold (less than 40xc2x0 F. (4xc2x0 C.)) through the bottom of the tank. The butane is pumped next through the bottom port, so it is immediately diluted so that surface vapor pressure is minimized to prevent evaporative losses. Alternatively, two or more of the MTHF, hydrocarbons and n-butane, if used, may be pumped through the bottom port together. If not blended at the distribution rack, the two or three components may be obtained as a blend through conventional gasoline pipelines. Because ethanol alone would otherwise raise the vapor pressure of the hydrocarbons and promote evaporative loss, the ethanol is preferably blended last, after the MTHF and n-butane, if present, has already blended with the hydrocarbon, by conventional splash blending techniques for the introduction of ethanol to motor fuels.
Thus, for a blend containing n-butane, ethanol, MTHF and pentanes plus, the MTHF is first pumped into the blending tank. Without agitation, pentanes-plus is pumped through the bottom of the tank into the MTHF, followed by the n-butane (if used). Finally, ethanol is blended through the bottom. The blend is then recovered and stored by conventional means.
The hydrocarbons, fuel grade alcohol and co-solvent are added in amounts selected to provide a motor fuel composition with a minimum anti-knock index of 87 as measured by ASTM D-2699 and D-2700 and a maximum DVPE of 15 psi (1 atm.) as measured by ASTM D-5191. A minimum anti-knock index of 89.0 is preferred, and a minimum anti-knock index of 92.5 is even more preferred. In the summer, a maximum DVPE of 8.1 psi (0.55 atm.) is preferred, with a maximum DVPE of 7.2 psi (0.5 atm.) being more preferred. In the winter, the DVPE should be as close as possible to 15 psi (1 atm), preferably between about 12 psi(0.8 atm.) and about 15 psi (1 atm.). For this reason, n-butane may be added to the motor fuel compositions of the present invention in an amount effective to provide a DVPE within this range.
In preferred motor fuel compositions in accordance with the present invention, the hydrocarbon component consists essentially of one or more hydrocarbons obtained from NGLs, blended with ethanol, MTHF and, optionally, n-butane. The NGLs hydrocarbons may be present at a level between about ten and about 50 percent by volume, the ethanol may be present in an amount between about 25 and about 55 percent by volume, the MTHF may be present in an amount between about 15 and about 55 percent by volume, and the n-butane may be present in a level between zero and about 15 percent by volume. More preferred motor fuel compositions contain from about 25 to about 40 percent by volume of pentanes plus, from about 25 to about 40 percent by volume of ethanol, from about 20 to about 30 percent by volume of MTHF and from zero to about ten percent by volume of n-butane.
The compositions of the present invention may be formulated as summer and winter fuel blends having T10 and T90 values as measured by ASTM-D86 within ASTM specifications for summer and winter fuel blends. The winter blend compositions of the present invention are significantly more volatile than conventional gasoline to aid cold weather starting. The T90 values indicate the amount of xe2x80x9cheavy-endxe2x80x9d components in the fuel. These substances are considered to be a primary source of unburned hydrocarbons during the cold start phase of engine operation. The lower values of xe2x80x9cheavy-endxe2x80x9d components in the compositions of the present invention also indicates superior emissions performance. The amount of solid residue after combustion is only one-fifth that typically found in conventional gasoline.
A particularly preferred summer fuel blend contains about 32.5 percent by volume of pentanes plus, about 35 percent by volume of ethanol, and about 32.5 percent by volume of MTHF. This blend is characterized as follows:
A particularly preferred winter fuel blend contains about 40 percent by volume of pentanes plus, about 25 percent by volume of ethanol, about 25 percent by volume of MTHF and about 10 percent by volume of n-butane. This blend is characterized as follows:
A preferred summer premium blend contains about 27.5 percent by volume of pentanes plus, about 55 percent by volume of ethanol and about 17.5 percent by volume of MTHF. The blend is characterized as follows:
A preferred winter premium blend contains about 16 percent by volume of pentanes plus, about 47 percent by volume of ethanol, about 26 percent by volume of MTHF and about 11 percent by volume of n-butane. The blend is characterized as follows:
Thus, it will be appreciated that the present invention provides a motor gasoline alternative essentially free of crude oil products that can fuel a spark ignition internal combustion engine with minor modifications, yet can be blended to limit emissions resulting from evaporative losses. The present invention provides fuel compositions containing less than 0.1 percent benzene, less than 0.5 percent aromatics, less than 0.1 percent olefins and less than ten ppm sulfur. The following examples further illustrate the present invention, and are not to be construed as limiting the scope thereof. All parts and percentages are by volume unless expressly indicated to be otherwise and all temperatures are in degrees Fahrenheit.